Hollow polymer particles are generally used in applications such as water-based paints and paper coating compositions as masking agents. Hollow polymer particles are also used as organic pigments having good optical properties in terms of opacity, whiteness, and the like due to higher light diffusivity, and hence, lower light transmittance, compared to polymer particles that are densely and uniformly filled with polymer.
In such applications, it is desired that a hollow polymer particle to be used possess an increased porosity to reduce the weight of paint, coated paper, or the like, and improved heat insulation and opacification. Further, a hollow polymer particle is also desired to have a small amount of aggregates, and to have higher polymerization stability, which permits such a hollow polymer particle having a high porosity to be efficiently and stably produced.
For example, Patent Literature 1 discloses that a hollow polymer particle having a high porosity is efficiently and stably obtained by performing production of a hollow polymer particle in the presence of a chain transfer agent.
Here, a hollow polymer particle is usually stored in a tank, a drum can, an 18-liter can, or the like after production. However, depending on production conditions of the hollow polymer particle, a problem may arise with respect to storage stability in that, for example, fungi may grow during storage, and emit unpleasant odor.
Patent Literature 2 discloses that a composition of a hollow polymer particle is produced by adding a predetermined amount of 2-methyl-4-isothiazolin-3-one and a predetermined amount of 2-n-octyl-4-isothiazolin-3-one to the hollow polymer particle, after production of the hollow polymer particle.
However, if preservatives are added after production of the hollow polymer particle as disclosed in Patent Literature 2, a sufficient degree of polymerization stability has not been achieved due to difficulties in lowering aggregates sufficiently. Moreover, in some cases, storage stability of such hollow polymer particle is not high enough.